Ankara, which has long backed rebels seeking to overthrow President Bashar al-Assad, and Moscow, Assad’s principal foreign ally, brokered the deal last month to create a demilitarized zone in the northwest Idlib region.
Idlib and adjacent areas are the last stronghold of the rebels, who rose up against Assad in 2011. The area is home to an estimated 3 million people, more than half of whom have already fled other areas as government forces advanced.
Shelling in Idlib killed at least seven civilians on Friday, the largest one-day loss of life there since Russian air strikes stopped in mid-August, a war monitor said.
Russian President Vladimir Putin, German Chancellor Angela Merkel, French President Emmanuel Macron and Erdogan are due to have four-way talks in Istanbul on Saturday. Putin spoke ahead of the summit by phone to Macron and Erdogan.
U.N. Syria envoy Staffan de Mistura, who is stepping down at the end of next month for family reasons, is also taking part.
Under their deal last month, Turkey and Russia agreed to set up a buffer zone running 15-20 km (9-13 miles) into rebel territory that had to be evacuated of all heavy weapons and all jihadist fighters.
